Existing methods to produce preforms for optical fibers are divided into two categories:
"external" methods making use of flame hydrolysis techniques, and
"internal" methods making use of a vapor phase oxidation reaction.
These existing internal methods use an internal vapor phase deposit inside a thin-walled silica tube (such as a tube with dimensions of 19.times.25, that is a tube whose internal diameter is 19 mm and external diameter is 25 mm) and have at least one of the following drawbacks:
optical fibers embodied from preforms obtained by these methods are relatively costly,
these optical fibers do not have extremely accurate geometrical characteristics, which in particular impairs the development of methods for connecting optical fibers, these methods being simple and less expensive to implement.
In particular, there exists a method frequently used for the production of preforms for optical fibers.
This method, known as a "modified chemical vapor deposition" method or MCVD, consists of coating the internal face of a thin-walled silica tube (conventionally a tube with dimensions of 19.times.25) with a suitable vitreous coating by means of a vapor phase oxidation reaction.
The tube and the vitreous coating formed inside this tube are slightly thick owing to the heating device used for attaining a sufficient and localized temperature for the oxidation reaction.
This heating device is a welding torch which actually has two drawbacks.
One first drawback is the difficulty of heating the inside of the tube through the dielectric of this tube by the heating device in question, whereas in order to deposit a good quality vitreous film, it is necessary to attain a sufficient temperature inside the tube without at the same time inducing an area contraction or collapse of the tube prejudicable to the proper laying of the deposit at this stage of the MCVD method.
One second drawback resides in the fact that the silica outside the tube evaporates when in contact with the flame of the welding torch when implementing the MCVD method.
So as to slightly mitigate these drawbacks and increase the capacity of a preform obtained by this MCVD method, an internal counter-pressure is generally used, which prevents any premature subsidence of the tube and thus makes it possible:
to use tubes whose internal diameter is slightly larger than the one indicated earlier and is, for example, 26 mm, hence a larger tube section for a given dielectric thickness
and/or depositing an internal vitreous film whose thickness is slightly larger that the admissible thicknesses with the "ordinary" MCVD method mentioned above.
Despite these improvements, the MCVD method can only be implemented with thin-walled tubes.
Another known technique to increase the capacity of the produced preform and thus lower the production cost of optical fibers from this preform consist of:
treating a first tube by the MCVD method and of then carrying out the operation for contracting or collapsing the area of this first tube and then attaching a second silica tube to this first tube and then of simultaneously contracting the area of the two tubes.
This other known technique nevertheless remains one delicate to implement and is relatively limited by the thickness of the deposit in the first tube.
There is also another technique which, after collapsing the area of the tube containing the internal deposit, consists of "recharging" the bar obtained, for example with the aid of a plasma torch able to produce by direct fusion vitrified silica via the projection of grains of natural silica, or is able to embody a deposit of films of synthetic pure silica or doped by fluorine.
Apart from the fact that such operations do not always improve efficiency as they involve tool changes, they are generally cumbersome to implement and still have the drawback of starting with a thin-walled tube, a tube which is generally produced by extrusion or ingot drawing systems whose geometrical precision is relatively slight.
The same limitations appear when other heating means are used through the thin-walled tube.
This applies particularly to the PCVD method ("Plasma Chemical Vapor Deposition") using a plasma generated by a microwave cavity, the only difference being the capacity (allowed by the fact that the depositing takes place at a lower temperature than in the MCVD method) for embodying extremely thin vitreous films and doping more easily these films with fluroine, the combination of these two effects opening more possibilities for embodying accurate and varied optical index profiles.
The document FR-A-2 600 also describes another method for producing a preform for optical fibers.
This other known method has the advantage of starting with pierced high precision wide bars.
However, it does have the disadvantage of embodying the internal deposit with the aid of a plasma column maintained by a direct wave injected with the aid of a ultrahigh frequency coupler connected to a microwave generator.
In fact, if the injection of this direct wave and obtaining of the plasma column, whose length may be modulated by modulating the electric power of the microwave generator used, are advantageous as they are relatively independent of the thickness of the dielectric of the pierced bar, it still remains difficult to obtain:
a correct longitudinal regularity for the deposition of thin films similar to those obtained by the PCVD method, and
a suitable longitudinal regularity of the desired optical index for these films.
Moreover, the length of the deposit is limited by the length of the plasma column which itself is linked to the power of the generator.
Thus, this other known method does have drawbacks, namely:
obtaining a deposit whose geometrical regularity is relatively poor and an optical index profile whose regularity is also relatively poor, which unfortunately is opposed to the advantage of starting with high-precision pierced bars, and
obtaining preforms whose capacity is relatively highly limited, which affects the economic advantage of the method.